Which reaction is exothermic?

A. [tex] CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O + 891 \, \text{kJ} [/tex]

B. [tex] 2 H_2O + 286 \, \text{kJ} \rightarrow 2 H_2 + O_2 [/tex]

C. [tex] N_2 + O_2 + 181 \, \text{kJ} \rightarrow 2 NO [/tex]



Answer :

To determine which reaction is exothermic, we need to analyze the energy changes associated with each chemical equation:

1. Reaction 1: [tex]\( CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O + 891 \text{ kJ} \)[/tex]
- This reaction releases 891 kJ of energy, indicated by the positive sign after the products.
- Since energy is released during the reaction, this reaction is exothermic.

2. Reaction 2: [tex]\( 2 H_2O + 286 \text{ kJ} \rightarrow 2 H_2 + O_2 \)[/tex]
- This reaction requires an input of 286 kJ of energy for it to proceed.
- Since energy is absorbed, this reaction is endothermic.

3. Reaction 3: [tex]\( N_2 + O_2 + 181 \text{ kJ} \rightarrow 2 NO \)[/tex]
- This reaction also requires an input of 181 kJ of energy.
- Since energy is absorbed, this reaction is endothermic.

Therefore, the reaction of interest that is exothermic is the first reaction:

[tex]\[ \boxed{1} \][/tex]

In summary, the exothermic reaction is:
[tex]\[ CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O + 891 \text{ kJ} \][/tex]

The numerical answer is [tex]\( \boxed{1} \)[/tex].